1. Field of the Invention
The invention relates generally to the field of communications. More particularly, the invention relates to the correction of gain and phase errors in a quadrature modulator.
2. Discussion of the Related Art
Quadrature modulation techniques enable two independent signals to be combined at a transmitter, transmitted on the same transmission band, and separated at a receiver. The principle of quadrature modulation is that two separate signals, I and Q (In-phase and Quadrature phase), are modulated by using the same carrier wave frequency, but the carrier wave of signal Q is 90° out of phase with the carrier wave of signal I. After modulation, the resulting signals are summed and transmitted. Because of the phase difference, the I and Q signals can be separated from each other when the summed signal is demodulated at the receiver.
Unfortunately, in practical applications, effects of gain imbalance and local oscillator phase error cause errors in the mutual phase and amplitude balance of the I and Q signals. For example, in the case of a single tone input to the quadrature modulator, the modulator output usually contains an undesired tone referred to as the sideband. If the desired output is located at ωc+ω (where ωc is the carrier frequency), then the undesired sideband is located at ωc−ω. Gain imbalance and phase error lead to the degradation of error vector magnitude (EVM) of the transmitter and bit error rate (BER) performance at the receiver.
Until now, the requirements of providing a sideband suppression technique for quadrature modulator using magnitude measurements have not been met.